Transcript for High Levels of carbon monoxide in the Iowa Law Enforcement Academy

Now, to the emergency playing out. At least 75 people were rushed to the hospital, most of them students. They'd been sleeping, cadets, future officers, when there was a gas leak. Authorities near Des Moines, Iowa, fear what could have happened if a guard hadn't called for help. Here's ABC's Clayton Sandell now. Reporter: Tonight, first responders in training at the center of a real emergency. At about 5:54 this morning, our overnight security officer at the Iowa law enforcement academy detected a very strong odor of what she describes as gas. And she called 911. Reporter: Inside, 86 police cadets asleep in their dorms. Evacuations begin immediately. Found high levels of carbon monoxide. Reporter: Many of the cadets showing symptoms of carbon monoxide poisoning, including vomiting, headache and fatigue. 75 rushed to hospitals. Officials say the building has a history of similar issues and is due to be demolished next summer. It did not have a carbon monoxide detector. Carbon monoxide poisoning can be deadly, killing 400 Americans every year and sending 20,000 to the emergency room. And David, authorities urge people to get a carbon monoxide detector like this one, and make sure to get your heating systems, those gas fireplaces and water heaters inspected every year. Winter is the worst time of year for carbon monoxide deaths.

This transcript has been automatically generated and may not be 100% accurate.

One teenager has died and dozens of people were sickened after a massive carbon monoxide leak at an apartment building in New Jersey late Thursday. Seven of those treated for poisoning were first responders.